The present invention relates to organic optoelectronic devices, more particularly to organic light emitting devices (OLEDs) that are protected from environmental elements such as moisture and oxygen.
Organic light emitting devices (xe2x80x9cOLEDsxe2x80x9d), including both polymer and small-molecule OLEDs, are potential candidates for a great variety of virtual- and direct-view type displays, such as lap-top computers, televisions, digital watches, telephones, pagers, cellular telephones, calculators and the like. Unlike inorganic semiconductor light emitting devices, organic light emitting devices are generally simple and are relatively easy and inexpensive to fabricate. Also, OLEDs readily lend themselves to applications requiring a wide variety of colors and to applications that concern large-area devices.
One factor limiting the practical application of OLEDs is their susceptibility to environmental elements such as moisture and oxygen. Oxygen and moisture can produce deleterious effects on certain OLED structural components, such as reactive metal cathode components. Without protection, the lifetime of the devices can be severely limited. For example, moisture and oxygen are known to increase xe2x80x9cdark spot areasxe2x80x9d in connection with OLED structures. The organic materials utilized in a conventional OLED structure can also be adversely affected by environmental species such as water and oxygen. Approaches to protecting OLEDs from environmental elements include, as discussed below, providing the OLED with a protective layer or cover that has decreased permeability to moisture and/or oxygen.
In general, two-dimensional OLED arrays for imaging applications are known in the art and typically include an OLED display area that contains a plurality of active regions or pixels arranged in rows and columns. FIGS. 1A and 1B are simplified schematic representations (cross-sectional view) of OLED structures provided with a protective layer. The OLED structure shown in FIG. 1A includes a single active region 15 which includes an electrode region such as anode region 12, a light emitting region 14 over the anode region 12, and another electrode region such as cathode region 16 over the light emitting region 14. The active region 15 is disposed on a substrate 10. Barrier layer 20 disposed over active region 15 is provided to restrict transmission of oxygen and water vapor from an outer environment to the active pixel 15.
In one common OLED structure in accordance with FIG. 1A, light from the light emitting layer 14 is transmitted downwardly through the substrate 10. In such a xe2x80x9cbottom-emittingxe2x80x9d configuration, the substrate 10 and anode 12 are formed of transparent materials. The cathode 16 and barrier layer 20 need not be transparent in this configuration. Moreover, structures are also known in which the positions of the anode 12 and cathode 16 in FIG. 1A are switched as illustrated in FIG. 1B. Such devices are sometimes referred to as xe2x80x9cinverted OLEDsxe2x80x9d. In such an inverted OLED bottom-emitting configuration as illustrated in FIG. 1B, the cathode 16 and substrate 10 are formed of transparent materials, while the anode 12 and barrier layer 20 need not be transparent.
However, other OLED architectures are also known in the art, including xe2x80x9ctop-emittingxe2x80x9d OLEDs and transparent OLEDs (or xe2x80x9cTOLEDsxe2x80x9d). For top-emitting OLEDs, light from the light emitting layer 14 is transmitted upwardly through barrier layer 20. In a top-emitting configuration like that shown in FIG. 1A, the cathode 16 and barrier layer 20 are formed of transparent materials while the substrate 10 and anode 12 need not be transparent. In an inverted top-emitting OLED configuration based on a design like that shown in FIG. 1B, the anode 12 and barrier layer 20 are formed of transparent materials. In this configuration, the cathode 16 and substrate 10 need not be transparent.
For TOLEDs, in which light is emitted from both the top and bottom of the device, the substrate 10, anode 12, cathode 16 and barrier layer 20 are formed of transparent materials. TOLEDs can be based on a configuration such as that shown in either FIG. 1A or FIG. 1B. Other OLED structures are known in the art and are suitable for use with the invention disclosed herein.
It is known to provide composite barrier layers in the form of a multilayer structure comprising an alternating series of one or more polymeric xe2x80x9cplanarizingxe2x80x9d sublayers and one or more xe2x80x9chigh densityxe2x80x9d sublayers of inorganic or dielectric material. Such a multilayer structure can be applied directly onto the substrate or active region of an OLED device by use of a polymer multilayer process or (xe2x80x9cPMLxe2x80x9d process). The PML process is disclosed, for example, in U.S. Pat. Nos. 4,842,893, 4,954,371, 5,260,095 and 6,224,948, all of which are incorporated herein in their entireties.
FIG. 2 shows a PML composite barrier layer 22 disposed on a top surface 11 of a substrate 10. The individual sublayers comprising composite barrier layer 22 are not shown in FIG. 2. The OLED structure shown in FIG. 2 includes a single active region 15 which includes an electrode region such as anode region 12, a light emitting region 14 over the anode region 12, and another electrode region such as cathode region 16 over the light emitting region 14. An additional barrier layer 20 is disposed over the active region 15, which can also be a multilayer structure, if desired. Composite barrier layer 22 is disposed on a top surface 11 of substrate 10, such that composite barrier layer 22 is positioned between substrate 10 and active region 15.
The PML process is advantageous because it is a vacuum compatible process which produces a conformal coating that does not require the separate attachment of a preformed multi-layer cover as is disclosed in the prior art for protecting an OLED from environmental elements. Moreover, the PML process produces a composite barrier layer with good resistance to moisture and oxygen penetration. The use of a PML composite barrier layer disposed on a substrate is particularly advantageous when the substrate is permeable to oxygen and moisture, as is often the case with polymeric substrates used in constructing flexible OLEDs (FOLEDs). Examples of OLEDs protected with a PML composite barrier layer are disclosed in, for example, U.S. Pat. Nos. 5,757,126, 6,146,225 and 6,268,695 all of which are incorporated herein in their entireties.
A typical PML composite barrier layer comprises a polymeric planarizing sublayer, which can be disposed over the OLED active region or over a surface of the substrate. A high-density sublayer is then disposed over this polymeric planarizing sublayer to form a first pair of sublayers. If desired, one or more additional pair(s) of sublayers may be deposited on the first pair of sublayers to provide a composite barrier layer comprising an alternating series of two or more polymeric planarizing sublayers layers and two or more high-density sublayers.
In certain OLED applications, such as bottom-emitting flexible OLEDs (FOLEDs), it is desirable that the substrate comprise a flexible and transparent polymeric material, such as polyethylene terephthalate (PET). Such an OLED can be protected from environmental elements by disposing a PML composite barrier layer 22 over a top surface 11 of the substrate 10 as shown in FIG. 2, such that the composite barrier layer is positioned between the substrate 10 and the active region 15. In FIG. 2, the polymeric planarizing sublayer contacts substantially the entire top surface 11 of the substrate 10. In general, it can be difficult to achieve adequate adhesion of one polymeric material to another polymeric material. Therefore, one difficultly presently associated with the use of a composite barrier layer, such as one formed by a PML process, is to effect adequate adhesion of a polymeric planarizing sublayer to a polymeric substrate. If inadequate adhesion is not obtained, separation or delamination of the composite barrier layer from the substrate may result. Partial or complete delamination renders the composite barrier layer substantially less effective to provide the OLED device with protection from environmental elements. It can also be difficult to effect adequate adhesion of a polymeric planarizing sublayer to an adjacent high-density sublayer or to a portion of the active region to which a composite barrier layer may be directly applied. This can also substantially compromise the effectiveness of the composite barrier layer to protect the OLED device from environmental elements such as moisture and oxygen.
In accordance with the foregoing it would, therefore, be desirable to provide an OLED device with a composite barrier layer in which a polymeric planarizing sublayer thereof has improved adhesion to a substrate, to an adjacent high-density sublayer and/or to the active region.
These and other challenges are addressed by the present invention which, in one embodiment, provides an OLED device comprising (a) a substrate; (b) an active region positioned over the substrate, wherein the active region comprises an anode layer, a cathode layer and a light-emitting layer disposed between the anode layer and the cathode layer; (c) a composite barrier layer disposed over the active region, over a surface of the substrate or both over the active region and over a surface of the substrate, the composite barrier layer comprising an alternating series of one or more polymeric planarizing sublayers and one or more high-density sublayers; and (d) a thin carbon layer disposed between at least one polymeric planarizing sublayer and a region of the OLED device selected from the group consisting of the substrate, an adjacent high-density sublayer, and the active region. The thin carbon layer has a thickness of preferably from about 3 to about 500 xc3x85.
In some preferred embodiments, the OLED device is a flexible OLED (FOLED) that incorporates a flexible and transparent polymeric substrate. In these embodiments, it is preferred to incorporate the thin carbon layer at least on a surface of the substrate that would otherwise be in direct contact with a polymeric planarizing sublayer of the composite barrier layer.
In other preferred embodiments, the composite barrier layer is provided as a conformal protective layer disposed at least over the OLED active region, and over at least a portion of the substrate surface. One preferred composite barrier layer will encapsulate the OLED device, such that the composite barrier layer covers all surfaces of the OLED device, including the active region and substrate surfaces, that would otherwise be exposed to the surrounding environment.
In another embodiment, the present invention provides an OLED device comprising (a) a substrate; (b) an active region positioned over the substrate, wherein the active region comprises an anode layer, a cathode layer and a light-emitting layer disposed between the anode layer and the cathode layer; (c) a composite barrier layer disposed over the active region, over a surface of the substrate, or both over the active region and over a surface of the substrate, the composite barrier layer comprising an alternating series of one or more polymeric planarizing sublayers and one or more high-density sublayers; wherein a surface of the substrate or of at least one of the polymeric planarizing sublayers has been subjected to a plasma treatment or chemical etching treatment to provide a roughened surface. In this embodiment, particularly where the substrate comprises a polymeric material, such as a transparent and flexible polymeric material useful in FOLEDs, it is preferable to provide the device with at least a roughened substrate surface to improve adhesion of a polymeric planarizing sublayer of a composite barrier layer to the substrate surface.
In any embodiment of the present invention, it may be preferred to provide a composite barrier layer comprising an alternating series of two or more polymeric planarizing sublayers and two or more high-density sublayers.
Preferred substrate materials include ceramic materials such as glasses, silicon-based materials, polymeric materials and metallic materials.
These and other embodiments and advantages of the present invention will become readily apparent to those of ordinary skill in the art upon review of the disclosure to follow.